Sandwich panels are frequently used in aviation, rocketry, and other applications where high strength and low weight are substantial considerations. As used herein, a sandwich panel includes any panel having an exterior skin attached on both sides of a core material. The skin is usually more rigid and dense than the core and is generally, but not always, made of metal or composite. However, different compositions or types of skin may be used on each side of the panel. The core is generally less stiff or less dense than the skin and may consist of, for example, a honeycombed metal structure, a plastic filler, or foam.
Due to their high strength-to-weight ratio and their durability, sandwich panels often form a portion of the exterior surface of large rockets. For example, in liquid fueled rockets, a sandwich panel is frequently formed into the shape of a cylinder and used to create a "dry bay" between one tank structure containing the oxidizer and another tank structure containing the fuel. Specifically, assuming the oxidizer tank is on top of the fuel tank, the sandwich panel cylinder is placed on top of the fuel tank structure with the bottom edge of the sandwich panel cylinder attached to the top of the fuel tank structure. The oxidizer tank structure is then placed on top of the sandwich panel cylinder with the top edge of the cylinder attached to the bottom of oxidizer tank.
In the example given for the liquid-fueled rocket, the sandwich panel is joined at its top edge to the oxidizer tank structure and at its bottom edge to the fuel tank structure. As the rocket accelerates during launch, both of these joints will obviously be subject to a substantial compressive load which tends to push the panel and each of the respective tank structures together. In addition, as the rocket rolls over from a vertical orientation at lift-off to a more horizontal orientation during flight, the different bending forces acting on the rocket may subject all or a portion of each joint to a moderate tensile load which tends to pull the joint apart.
The methods commonly used to join a sandwich panel to another structure, such as the fuel tank or oxidizer tank in the rocket example, generally feature either shear joints or pan-down joints. In a shear joint, a flange or other member attached to the structure overlaps either the inner or outer skin of the sandwich panel. Bolts or other fasteners pass through both the flange and the panel in a direction normal to the surface of the skin and draw the panel and flange together. The bolts or fasteners must be longer than the combined width of the panel and the flange. In the previously described example of a liquid-fueled rocket, these bolts or fasteners transmit the compressive load from the panel to the flange and a large number of bolts or fasteners are required at each joint.
A pan-down joint is similar to a shear joint, except the panel is fabricated so it is thinner near the edge of the panel to be joined with the structure. For example, the core of the panel near that edge can be machined thinner and the skin of the panel formed to conform with the shape of the machined core. The panel is then attached to the flange using bolts or other fasteners in the same manner described above for shear joints, but with the bolt or fastener passing through the thinner portion panel. As a result, the bolts or fasteners do not need to be as long as those required by shear joints. However, pan-down joints are generally more expensive to make since the core must be machined thinner near the edges and the skin must be formed to conform with the core. In addition, since the core is thinner near the edges, the panel strength or efficiency is reduced near the edge of the panel. Finally, since the compressive load is still transmitted through the bolts or fasteners, a large number of bolts or fasteners are still required.
In both shear joints and pan-down joints, the principal load path for both a compressive load and a tensile load is not a straight line. Instead, the principal load path "jogs" as it passes from the panel to the flange. In addition, due to the large number of relatively long bolts or fasteners required, both joints add substantial weight to the overall apparatus. Moreover, the ability of the joint to bear a compressive load is dependent upon the integrity of the bolts or fasteners since a compressive load is not transferred in bearing, i.e., across a surface which is normal to the direction of the load force, but is instead transferred through the bolts or fasteners. In addition, in both shear joints and pan-down joints, eccentricities in the shape of either the panel or the flange can substantially reduce the efficiency of the joint. Finally, both shear joints and pan-down joints to reduce the overall strength or efficiency of the panel, particularly in the pan-down joints where the core is thinner near the edges.